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Date:   Wed, 24 Jul 2019 15:31:03 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Nitesh Narayan Lal <nitesh@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>,
        kvm@vger.kernel.org, dave.hansen@intel.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, yang.zhang.wz@gmail.com,
        pagupta@redhat.com, riel@surriel.com, konrad.wilk@oracle.com,
        lcapitulino@redhat.com, wei.w.wang@intel.com, aarcange@redhat.com,
        pbonzini@redhat.com, dan.j.williams@intel.com,
        alexander.h.duyck@linux.intel.com
Subject: Re: [PATCH v2 0/5] mm / virtio: Provide support for page hinting
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On Wed, Jul 24, 2019 at 08:41:33PM +0200, David Hildenbrand wrote:
> On 24.07.19 20:40, Nitesh Narayan Lal wrote:
> > 
> > On 7/24/19 12:54 PM, Alexander Duyck wrote:
> >> This series provides an asynchronous means of hinting to a hypervisor
> >> that a guest page is no longer in use and can have the data associated
> >> with it dropped. To do this I have implemented functionality that allows
> >> for what I am referring to as page hinting
> >>
> >> The functionality for this is fairly simple. When enabled it will allocate
> >> statistics to track the number of hinted pages in a given free area. When
> >> the number of free pages exceeds this value plus a high water value,
> >> currently 32,
> > Shouldn't we configure this to a lower number such as 16?
> >>  it will begin performing page hinting which consists of
> >> pulling pages off of free list and placing them into a scatter list. The
> >> scatterlist is then given to the page hinting device and it will perform
> >> the required action to make the pages "hinted", in the case of
> >> virtio-balloon this results in the pages being madvised as MADV_DONTNEED
> >> and as such they are forced out of the guest. After this they are placed
> >> back on the free list, and an additional bit is added if they are not
> >> merged indicating that they are a hinted buddy page instead of a standard
> >> buddy page. The cycle then repeats with additional non-hinted pages being
> >> pulled until the free areas all consist of hinted pages.
> >>
> >> I am leaving a number of things hard-coded such as limiting the lowest
> >> order processed to PAGEBLOCK_ORDER,
> > Have you considered making this option configurable at the compile time?
> >>  and have left it up to the guest to
> >> determine what the limit is on how many pages it wants to allocate to
> >> process the hints.
> > It might make sense to set the number of pages to be hinted at a time from the
> > hypervisor.
> >>
> >> My primary testing has just been to verify the memory is being freed after
> >> allocation by running memhog 79g on a 80g guest and watching the total
> >> free memory via /proc/meminfo on the host. With this I have verified most
> >> of the memory is freed after each iteration. As far as performance I have
> >> been mainly focusing on the will-it-scale/page_fault1 test running with
> >> 16 vcpus. With that I have seen at most a 2% difference between the base
> >> kernel without these patches and the patches with virtio-balloon disabled.
> >> With the patches and virtio-balloon enabled with hinting the results
> >> largely depend on the host kernel. On a 3.10 RHEL kernel I saw up to a 2%
> >> drop in performance as I approached 16 threads,
> > I think this is acceptable.
> >>  however on the the lastest
> >> linux-next kernel I saw roughly a 4% to 5% improvement in performance for
> >> all tests with 8 or more threads. 
> > Do you mean that with your patches the will-it-scale/page_fault1 numbers were
> > better by 4-5% over an unmodified kernel?
> >> I believe the difference seen is due to
> >> the overhead for faulting pages back into the guest and zeroing of memory.
> > It may also make sense to test these patches with netperf to observe how much
> > performance drop it is introducing.
> >> Patch 4 is a bit on the large side at about 600 lines of change, however
> >> I really didn't see a good way to break it up since each piece feeds into
> >> the next. So I couldn't add the statistics by themselves as it didn't
> >> really make sense to add them without something that will either read or
> >> increment/decrement them, or add the Hinted state without something that
> >> would set/unset it. As such I just ended up adding the entire thing as
> >> one patch. It makes it a bit bigger but avoids the issues in the previous
> >> set where I was referencing things before they had been added.
> >>
> >> Changes from the RFC:
> >> https://lore.kernel.org/lkml/20190530215223.13974.22445.stgit@localhost.localdomain/
> >> Moved aeration requested flag out of aerator and into zone->flags.
> >> Moved bounary out of free_area and into local variables for aeration.
> >> Moved aeration cycle out of interrupt and into workqueue.
> >> Left nr_free as total pages instead of splitting it between raw and aerated.
> >> Combined size and physical address values in virtio ring into one 64b value.
> >>
> >> Changes from v1:
> >> https://lore.kernel.org/lkml/20190619222922.1231.27432.stgit@localhost.localdomain/
> >> Dropped "waste page treatment" in favor of "page hinting"
> > We may still have to try and find a better name for virtio-balloon side changes.
> > As "FREE_PAGE_HINT" and "PAGE_HINTING" are still confusing.
> 
> We should have named that free page reporting, but that train already
> has left.

I think VIRTIO_BALLOON_F_FREE_PAGE_HINT is different and arguably
actually does provide hints.

> -- 
> 
> Thanks,
> 
> David / dhildenb